A colourful description of how Winston Churchill spent a night drinking with Joseph Stalin in Moscow during a wartime meeting is contained in a letter from Sir Alexander Cadogan, a top official at the Foreign Office, contained in a highly classified document released on Thursday.

He described how at mealtimes between the meetings in August 1942, Churchill "engaged the company in irreverent and irresponsible discourse".

Cadogan's account is contained in a letter to Lord Halifax, a former foreign secretary, when he was UK ambassador in Washington in the late 1940s.

"Nothing can be imagined more awful than a Kremlin banquet, but it has to be endured," wrote Cadogan.

"Unfortunately Winston didn't suffer it gladly. However, next morning he was determined to fire his last bolt and asked for a private talk, alone with Stalin. This was fixed for 7pm."

Cadogan said he was himself summoned to Stalin's room. "There I found Winston and Stalin … sitting with a heavily laden board between them: food of all kinds crowned by a suckling pig, and innumerable bottles."

Cadogan continued: "What Stalin made me drink seemed pretty savage: Winston, who by that time was complaining of a slight headache, seemed wisely to be confining himself to a comparatively innocuous effervescent Caucasian red wine."

Cadogan added that "everything seemed to be as merry as a marriage-bell" as Stalin went on about the benefits of the Soviet system. The party broke up at 3am.